A recent vehicle such as an automobile is designed so as to minimize engine sound and sounds generated as a result of travel of the vehicle, to thereby make occupants more comfortable and to prevent radiation of noise to the outside of the vehicle. However, such design brings about a problem in that the relation between the actual traveling conditions (vehicle speed, etc.) and the sound heard by the occupants becomes weak, and drive feeling deteriorates. In order to solve such a problem, there has been developed a vehicle sound generation apparatus which causes a speaker to produce an engine sound toward the interior of the vehicle cabin without radiating any sound to the outside of the vehicle. Such an apparatus is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2008-13064.
This vehicle sound generation apparatus (operating sound transmission apparatus) includes a sound pressure sensor disposed at an intake air introduction port or inside an intake pipe of an engine; a rotary pulse sensor for detecting ignition pulses of the engine; an accelerator opening sensor for detecting an amount by which an accelerator pedal is pressed down; a speaker and a controller which are disposed in the engine compartment or the cabin and produces an operating sound of the engine. The controller includes an order filter which changes a sound wave signal output from the sound pressure sensor and passing therethrough, in accordance with an input frequency from the rotary pulse sensor; and a level adjustor which increases and decreases the level of the sound wave signal having passed through the order filter in accordance with an opening signal from the accelerator opening sensor. The sound wave signal output from the controller is amplified by an amplifier and is output from the speaker.
However, in the case of the above-described conventional vehicle sound generation apparatus, a plurality of sound pressure sensors are attached to an external air introduction port of an air cleaner and an external air introduction port of an air duct disposed on the upstream side of the air cleaner. Or, a plurality of sound pressure sensors are attached to an intake manifold. Therefore, in the case where sound pressure sensors are attached to the air cleaner and the external air introduction port of the air cleaner, the detection of the sound pressure of intake sound is difficult, because the intake sound is of low level. As a result, an apparatus for accurately reproducing intake sound is required. In the case where sound pressure sensors are attached to the intake manifold, since the sound pressure sensors are located near the engine, which generates explosion sound, the variation of pulsation becomes large, which raises a problem in that the sound produced by the speaker becomes rough and irritating.
Moreover, in the case where microphones are used as sound pressure sensors and are attached to the air cleaner and the external air introduction port of the air cleaner, the microphones detect external sounds in addition to intake sound, which raises a problem of difficulty in obtaining a desired engine intake sound. Also, there arises a problem in that it is difficult for microphones to maintain their performances over a long period of time in a severe environment in the engine compartment in which parts must have durability against heat, water, oil, and dust.